The present invention relates to disc drives. More specifically, the present invention relates to a servo test used in certifying, or qualifying, a disc drive.
Disc drives illustratively include data transducers located relative to disc surfaces of rotatable discs in a disc stack. The data transducers are provided with a write signal to encode data on the disc surface. When the disc surface is moved relative to the data transducer, the data transducer generates a read signal indicative of data which has already been encoded on the disc.
In order to write data to the disc, a servo system is used to position the data head at one of a plurality of concentric tracks on the disc surface. A disc drive controller then provides information which generates the write signal that is provided to the data transducer. The data transducer thus encodes data on the disc surface at the desired track location.
When a read operation is to be performed, the servo system again positions the data transducer relative to a desired track on the disc surface. The data transducer then generates a read signal indicative of information encoded on the track over which the data transducer is positioned. This information is provided back to the drive controller which identifies data based on the read signal received.
It can thus be seen that, in order to perform a read or write, the servo system must perform a track following operation. In a track following operation, the servo system holds the data transducer over a track on the disc surface, while the disc rotates, to read data from, or write data to, the disc surface. In order to access a desired portion of the disc surface, the servo system must perform a track seek operation. In the track seek operation, the servo system moves the data transducer radially relative to the disc surface to a desired one of the concentric tracks to be accessed.
Before disc drives are certified for sale by many manufactures, they are subjected to a variety of tests. A servo test is used by many manufactures as part of the hard disc drive certification test process. The servo test is used because the data transducer must be able to be positioned accurately relative to the track prior to storing data on and retrieving data from the disc.
There is a desire to obtain ever increasing storage capacity on disc drives. There is also a desire to reduce the size of disc drives. One of the ways in which both of these objectives are accomplished is to increase the number of tracks per inch (TPI) on the disc surface. Two of the known problems which must be overcome to increase tracks per inch are repeatable run-out (RRO) and non-repeatable run-out (NRRO). One of the sources for repeatable run-out can be eccentric rotation of the disc drive spindle which can be caused by mechanical misalignments or tolerance stack up, or which can be caused by excitation of resonant modes in the disc drive. RRO can cause position error samples (PES) to be written in the wrong position and lead to positioning the data transducer off of the track center.
One of the technologies used to address this written in repeatable run-out, and thus to increase TPI, is referred to as zero acceleration path (ZAP) learning. Zap learning is known, and need not be fully recounted here. Briefly, however, ZAP learning involves an extended learning process in which a highly accurate positioning system detects repeatable run-out and writes in position error information in order to compensate for the repeatable run-out for subsequent servo positioning during operation of the disc drive. However, ZAP learning takes a long time, typically on the order of several hours, depending upon the number of discs in the drive. This is because ZAP learning may require many revolutions per track. ZAP learning can also be highly susceptible to external vibrations and non-repeatable run-out. In other words, when the disc drive is subjected to external vibrations and non-repeatable run-out, ZAP learning may inadvertently result in a disc drive that exhibits worse servo performance than one which was not subjected to ZAP learning at all.
In current products, the servo test is broken into several parts. All the surfaces are first scanned for bad tracks with a very loose position error sample (PES) threshold. Bad tracks are identified and avoided. This can take in excess of two hours. ZAP learning is then performed on all the tracks. This can take in excess of five hours. After ZAP learning has been performed on all the surfaces, all the surfaces are again scanned for bad servo sectors, with a tighter PES threshold than that used during the first scan. This can also take in excess of two hours. In sum, the current servo testing process can consume approximately seventy percent of a disc drive""s total test time.
The present invention addresses one or more of these deficiencies.
A servo test is performed on a disc drive. The servo test includes performing a position error sample (PES) threshold test on tracks in the disc drive and identifying those tracks that fail the PES servo sample test. Zero acceleration path (ZAP) learning is performed on the identified tracks, and is performed on less than all of the tracks on the disc drive. Another PES servo threshold test is performed on the identified tracks to determine whether they pass the servo test, given the ZAP learning process.